A sign on the door at a McDonald's restaurant in Chicago tells customers the location has stopped serving sliced tomatoes Monday.

McDonald's, Wal-Mart and other U.S. chains have halted sales of some raw tomatoes as federal health officials work to trace the source of a multistate salmonella food-poisoning outbreak.

Burger King, Outback Steakhouse and Taco Bell were among other restaurants voluntarily withdrawing tomatoes from their menus, following federal recommendations that consumers avoid red plum, red Roma or round red tomatoes unless they were grown in certain states and countries.

McDonald's Corp., the world's largest hamburger chain, has stopped serving sliced tomatoes on its sandwiches as a precaution until the source of the bacterial infection is known, according to a statement Monday from spokeswoman Danya Proud. McDonald's will continue serving grape tomatoes in its salads, because no problems have been linked to that variety, she said.

The source of the tomatoes responsible for the illnesses in at least 16 states, including Utah, has not been pinpointed. The Centers for Disease Control and Prevention has said at least 23 people have been hospitalized, and no deaths have been reported.

Locally, inspectors from Utah Department of Agriculture and Food contacted the 10 largest grocery store chains and food distributors in the state to confirm they had received the FDA warning. The sellers are Smith's Food and Drug, Albertsons, Associated Foods, Nicholas Foods, Sysco Corp., Wal-Mart/Sam's Club and Harmons, said Larry Lewis, spokesman for the department.

The stores and distributors were aware of the warning. None had found any of the raw tomatoes in question. Some of the stores are placing signs in their produce section indicating their tomatoes are not the ones in question.

"We recommend stores sell only tomatoes from states not associated with the salmonella outbreak," Lewis said.

Utah's single laboratory-linked case was of a man who had traveled to New Mexico but was sick when he returned to Utah, according to the Utah Department of Health.

In downtown Chicago, travel agent Connie Semaitis, 49, bought a cheeseburger and a drink at a McDonald's restaurant during lunch hour Monday. She said she was happy the chain was being cautious.

"I'd rather be safe than sorry," Semaitis said.

The Food and Drug Administration warned consumers in New Mexico and Texas as early as June 3 about the outbreak. The agency expanded its warning during the weekend, and chains began voluntarily removing many red plum, red Roma or round red tomatoes from their shelves in response.

Tampa-based OSI Restaurant Partners LLC, which owns and operates eight brands, including Outback Steakhouse, Carrabba's and Bonefish Grill, said it stopped serving all raw tomatoes other than grape tomatoes on Saturday evening. The company also instructed its restaurants to discard salsa and other prepared foods containing raw tomatoes.

Miami-based Burger King Corp. said it had withdrawn raw round red tomatoes from most of its U.S. restaurants. The company also removed the variety from all its locations in Canada and Puerto Rico and from some restaurants on other Caribbean islands.

Burger King said some California restaurants were allowed to continue using the tomatoes, because they buy from growers in states the FDA has said are not involved in the outbreak.

Orlando-based Darden Restaurants, which owns and operates six brands  including Red Lobster and Olive Garden  and Denver-based Chipotle Mexican Grill Inc. also said they'd halted serving tomatoes, with Chipotle posting a notice to customers on its Web site that its tomato salsa is temporarily unavailable.

Taco Bell Corp. pulled tomatoes as well, the Los Angeles Times reported. Messages seeking comment were left by The Associated Press for Yum Brands Inc., which is based in Louisville, Ky., and owns Taco Bell.

Wal-Mart Stores Inc., which is based in Bentonville, Ark., and is the largest grocery seller in the United States, repeated a statement Monday that some tomatoes had been removed from its shelves. Wal-Mart initially announced the action Thursday.

"According to the FDA, cherry tomatoes, grape tomatoes and tomatoes sold with the vine still attached are not affected by this incident, and may still be found on our shelves," Wal-Mart said. But the retailer, responding to concerns of federal officials, had stores in New Mexico, Texas and "other select stores with tomatoes from related sources" pull tomatoes listed in the FDA alert. The company said it took from its shelves certain Romas, slicers, three-pack and four-pack tomatoes.

Wal-Mart also programmed an electronic block into its cash registers so they could not ring up any of the tomatoes that may have remained on the shelves.

The FDA is investigating the source of the outbreak, agency spokeswoman Kimberly Rawlings said.

"We are working hard and fast on this one and hope to have something as quickly as possible," Rawlings said Monday.

Rawlings said the FDA's "traceback" investigations typically look at similarities in illnesses reported to the CDC by state health officials. Investigators work backward to find the source of the contaminated product.

Cherry tomatoes, grape tomatoes, tomatoes sold with the vine still attached and homegrown tomatoes are likely not the source of the outbreak, federal officials said.